1. Field of the Invention
The present invention relates to a multi-layer printed circuit board and more particularly, to a multi-layer printed circuit having low noise characteristics.
2. Description of the Related Art
Please refer to FIG. 1. FIG. 1 is shown a prior art printed circuit board. A printed circuit board 10 is composed of three different layers; one is for power, one is for ground, and the other is for components. A component layer (top side or bottom side) is used for the layout of signal line (as a signal layer). Electronic elements are placed on the top layer or the bottom layer of the printed circuit board 10, and signal pins, power pins and ground pins are respectively connected to the other electronic components on the component layer, the power layer, and the ground layer. Additionally, the electronic components can be divided into two groups, analog electronic components and digital electronic components. The analog electronic components may include an audio chip, an amplifier, a power supply, etc.; the digital electronic components may include a digital signal processor (DSP), a microprocessor, a video driver, etc. To avoid noise generated by the digital electronic components from affecting the analog electronic components, an isolation area 11 is usually separated from the power and/or ground layer of the printed circuit board 10. A portion within the isolation area 11 is called an analog electronic component area Al, and the other portion outside of the isolation area 11 is called the digital electronic component area A2. No connection exists between the analog electronic component area A1 and the digital electronic component area A2. Hence, noise generated by the digital electronic components cannot pass through the power layer to affect the analog electronic components, and the interference from the printed circuit board 10 is reduced significantly. Similarly, the ground layer of the printed circuit board 10 can have the same design.
However, when the connections between digital and the analog electronic components are required, the signal lines have to cross over the isolation area 11; the reference plane of the signal lines is not continuous and it will affect signal quality. Furthermore, the electromagnetic interferences between signals will become stronger and the electromagnetic radiations are likely to exceed EMC standards' limits. In a prior art, a metal neck line is used to connect the two isolation areas to provide continuous voltage levels, but the noise isolation between the areas becomes even lower near the resonant frequencies of the two isolation areas.
In a prior art technology for improving isolation effects, a decoupling capacitor is connected between the power layer and the ground layer. However, every capacitor has a series inductance, which provides effective filtering at low frequency regions but not at high frequency regions.
Therefore, it is desirable to provide a multi-layer printed circuit with low noise to mitigate and/or obviate the aforementioned problems.